Carrier for catalyst and method for preparing the same

ABSTRACT

It is to provide a catalyst carrier having large pore size and porosity and a small pressure loss. 
     The catalyst carrier is characterized by covering a surface of each particle in a silicon-containing ceramic carrier with alumina thin film, and is produced by immersing a carrier having an oxide film of a silicide in its surface in a solution of aluminum containing metal compound, drying by heating, calcining, subjecting to an immersion treatment in hot water and thereafter firing.

TECHNICAL FIELD

This invention relates to a catalyst carrier for the purification of anexhaust gas and a method of producing the same, and more particularlyproposes a catalyst carrier capable of efficiently conductingoxidation-removal of carbon monoxide (CO) and hydrocarbon (HC) includedin the exhaust gas and reducing-removal of nitrogen oxide (NOx) andbeing small in the pressure loss and having a high gathering efficiencyof diesel particulate.

BACKGROUND ART

Heretofore, there is a catalyst carrier for the purification of anexhaust gas from an automobile, e.g. a catalyst carrier for purifying anexhaust gas from a diesel engine as shown in FIGS. 1( a) and (b). As atypical example of such a catalyst carrier, there is a honeycomb typefilter 100 by making each cell 101 as a path for an exhaust gas in ahoneycomb shape from a porous silicon carbide sintered body havingexcellent heat resistance and thermal conductivity and alternatelyclogging these cells 101. This type of the honeycomb type filter 100 isconnected to an exhaust side of a diesel engine and has a structure thatPM (particulate matter) deposited on the filter, HC, CO and the like areremoved by oxidation decomposition.

As such a carrier for the catalyst, it is known that a carrying layermade of γ-alumina is formed on a surface of a filtering wall (cell wall)102 of a honeycomb-shaped heat-resistant carrier made of, for example,cordierite and a noble metal catalyst such as Pt, Pd, Rh or the like iscarried on the carrying layer. For example, JP-A-5-68892 discloses acatalyst carrier obtained by adding and mixing γ-alumina with aninorganic binder and pulverizing them to obtain a fine powder slurry andthen uniformly spraying the slurry onto a surface (wall face) of ahoneycomb filter made of cordierite to form so-called wash coat aluminalayer 103.

The alumina layer 103 formed by the conventional technique or washcoated (wash coat alumina layer) is shaped by a thin film uniformlycovering the wall face of the filtering wall 102 as shown in FIG. 2( a)and has a fine pore structure as shown by a partial enlarged view inFIG. 2( b). The pore size in such a fine pore structure is mainly 20–500angstrom and a specific surface area is usually 50–300 m²/g. And also,the alumina layer 103 acts as a catalyst carrying layer dispersinglysupporting a catalyst such as a noble metal or the like on its surface,so that it is required to enlarge the surface area and have a certainthickness (about 50–100 μm).

However, the wash coated alumina layer 103 is small in the pore size andporosity and large in the permeation resistance, so that there is aproblem that pressure loss considerably increases as compared with thecarrier having no alumina layer.

Furthermore, the wash coated alumina layer 103 is poor in the adhesionproperty because it is uniformly coated onto the surface of the carrieras a filtering wall 102. Therefore, when the deposited ash is cleanedafter the purification of the exhaust gas, the alumina layer 103 isfeared to be simply peeled. And also, there is a problem that the washcoated alumina layer 103 is poor in the heat resistance because such alayer has a fine pore structure as mentioned above but is as small as20–500 angstroms in the pore size and proceeds the sintering when beingexposed to a higher temperature for a long time to cause a phasetransformation into α-phase to lower the surface area. Furthermore,since the surface area is small, a distance between noble metalparticles carried on the alumina is small and hence the lowering of thesurface area is caused as the sintering proceeds and there is a problemof lowering the catalytic action itself.

As seen from the above, it is important that the surface of the carrier,particularly the surface of the alumina layer covering the surfacethereof is maintained at the high temperature for the long time as animportant property of the catalyst carrier.

It is, therefore, an object of the invention to propose a catalystcarrier being large in the pore size and porosity and small in thepressure loss though an alumina thin film is formed on the surface and amethod of producing the same.

It is another object of the invention to propose a catalyst carrierhaving a large surface area of an alumina layer as a catalyst carryinglayer and an excellent heat resistance and a method of producing thesame.

DISCLOSURE OF THE INVENTION

As means for solving the above problems, the invention adopts a catalystcarrier characterized in that a surface of each particle ofsilicon-containing ceramic carrier such as silicon carbide or siliconnitride is covered with a thin film of alumina.

The silicon-containing ceramic carrier is favorable to be constructedwith a carrier of silicide including a non-oxide ceramic such as siliconcarbide or silicon nitride and an oxide ceramic such as sialon, mulliteor cordierite.

The silicon-containing ceramic carrier is characterized by being any ofa porous body, fiber shaped body and pellet shaped body. Preferably, itis formed by a honeycomb-like porous silicon carbide sintered bodyhaving a SiO₂ layer on its surface wherein an amount of such a layeroccupied in the carrier is 0.001–20 wt %.

In the invention, it is a preferable embodiment that the alumina thinfilm covering each particle surface of the ceramic carrier indicates atransplant structure of bristling with fine fibers having a diameter:2–50 nm, a length: 20–300 nm and a ratio of total length/diameter of5–100 at a microscopic section and has a specific surface area of 50–300m²/g, and that the alumina thin film is an amount of 0.1–15 wt % per thecarrier as an alumina amount.

The catalyst carrier can be produced by forming an alumina thin film ona surface of a silicon-containing ceramic carrier through the followingsteps (a)–(e).

-   (a) Solution immersing step: the carrier is immersed in a solution    of aluminum containing metal compound.-   (b) Drying step: the carrier is heated and dried.-   (c) Calcining step: the carrier is heated and fired at a temperature    of not lower than 300–500° C. to form amorphous alumina thin film.-   (d) Heat treating step: the carrier is immersed in a hot water of    100° C. and dried.-   (e) Finish firing step: it is fired at 500–1200° C.

And also, another production method of the invention is characterized byforming an alumina thin film on a surface of a silicon-containingceramic carrier through the following steps (a)–(f).

-   (a) Preliminary treating step: the silicide ceramic carrier is    heated to a temperature of 1000–1500° C. to form an oxide film of    the silicide.-   (b) Solution immersing step: the carrier is immersed in a solution    of aluminum containing metal compound.-   (c) Drying step: the carrier is heated and dried.-   (d) Calcining step: the carrier is heated and fired at a temperature    of not lower than 300–500° C. to form amorphous alumina thin film.-   (e) Heat treating step: the carrier is immersed in a hot water of    100° C. and dried.-   (f) Finish firing step: it is fired at 500–1200° C.

Moreover, the component composition, structure and properties of thesilicon-containing ceramic carrier are as mentioned above in each of theabove production methods, and also the alumina thin film covering thesurface of each ceramic particle is the same as mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a typical embodiment of the catalystcarrier;

FIG. 2 is an outline of the conventional wash coat alumina layer;

FIG. 3( a) is an enlarged photograph of a filtering wall, (b) is anoutline thereof, and (c) is a diagrammatically section view of analumina thin film;

FIG. 4 is a schematic view illustrating a pressure loss characteristic;

FIG. 5 is an electron microphotograph showing a particle structure of acatalyst carrier;

FIG. 6 is a graph showing a pressure loss characteristic in an example;and

FIG. 7 is a graph showing a heat resistance of an alumina coat in anexample.

BEST MODE FOR CARRYING OUT THE INVENTION

A catalyst carrier 1 according to the invention is used by forming afiltering wall 2 with a porous silicon-containing ceramic sintered bodypreferably typifying silicon carbide (hereinafter referred to as porousSiC sintered body, which is not, of course, restricted thereto),covering a surface of the filtering wall 2, particularly each surface ofSiC particles with an alumina thin film 3 as a catalyst carrying layerat a given thickness, and carrying Pt or Pd on the alumina thin film 3.

As the catalyst carrier used in the invention can be used ones obtainedby kneading powder of an oxide ceramic such as silicon carbide powder,silicon nitride powder or the like, or powder of a silicon-containingceramic belonging to an oxide such as sialon, mullite, cordierite or thelike with an organic binder, a lubricant, a plasticizer and water andshaping through extrusion and sintering. In this way, a wall-flowhoneycomb type filter is formed as shown in FIG. 1.

An example using SiC sintered body as the silicon-containing ceramiccarrier is explained below.

The catalyst carrier (filter) 1 is constructed with SiC sintered bodywherein plural through-holes (cells) are regularly formed in anapproximately square shape at its section along an axial line directionthereof. The cells are separated from each other through filtering walls(hereinafter referred to as cell wall) 2, while opening portion of eachcell is sealed at its one edge face side with a sealing body 104 and theother edge face thereof is opened, so that the carrier as a whole has astructure that the opening portions and sealed portions of the cellsindicate a checkered pattern. In the carrier(filter) 1 made of the SiCsintered body are formed many cells 101 of a rectangular form atsection. In other words, the filter has a honeycomb structure.

Moreover, the density of the cells 101 is about 200 cells/square inch.That is, about a half of many cells is opened at their end faces ofupstream side and the remaining cells are opened at their end faces ofdownstream side, and the thickness of the cell wall 2 separating thecells 101 is set to be about 0.4 mm.

The thus obtained catalyst carrier made of SiC sintered body is aso-called wall-flow type structure partitioned by porous cell walls 2 asshown in FIG. 3( a). In this case, it is favorable that an average poresize in the pores of the porous cell wall 2 is within a range of 5 μm–15μm as measured by a mercury press-fitting process. When the pore size isrepresented by common logarithms, it is favorable that a value ofstandard deviation in the pore size distribution is not more than 0.20.

When the cell wall 2 has such a pore size, it is favorable to catch fineparticulates. That is, when the average pore size of the cell wall 2 isset to the above range, diesel particulates can surely be caught. On theother hand, when the pore size of the cell wall 2 is less than 5 μm, thepressure loss when the exhaust gas passes through the inner wall becomesextremely large and the stop of the engine is apt to be caused. Andalso, when the average value of the pore size exceeds 15 μm, fineparticulates can not be caught efficiently.

In the production of such a catalyst carrier, a starting material isformed, for example, by compounding 70 parts by weight of siliconcarbide powder having an average particle size of about 10 μm with about30 parts by weight of silicon carbide powder having an average particlesize of about 0.5 μm, about 6 parts by weight of methylcellulose as abinder based on 100 parts by weight of the ceramic powder, and about 25parts by weight of a dispersant of an organic solvent and water based on100 parts by weight of the ceramic powder. After kneading, thecompounded mixture is shaped into a honeycomb form by extrusion shaping.Thereafter, cells 101 are partly sealed into a checkered pattern. Then,the shaped body is dried and degreased and fired at 2200° C. in an inertatmosphere for 4 hours to obtain a desired catalyst carrier.

In the invention, a most characteristic feature lies in that thesurfaces of the inner walls of the catalyst carrier 1 or cell walls 2are covered with an alumina thin film. More concretely, the surface ofeach particle in the SiC sintered body constituting the cell wall 2 isindividually covered with the alumina thin film.

FIG. 2( b) shows a conventional technique that the surface of the cellwall 102 is evenly covered with the alumina layer 103 by a wash coatprocess. On the other hand, FIG. 3( b), (c) shows an embodiment of theinvention, wherein each surface of SiC particles 4 constituting the cellwall 2 is individually covered with the alumina thin film 3.

Thus, the film carrying the catalyst inherent to the invention is notsimply a case that the surface of the cell wall 2 as a filtering wallfor exhaust gas is uniformly covered with the alumina layer 103 as isusual, but is a case that the surface of each SiC particles 4constituting the cell wall 2 is individually covered with the aluminathin film 3. In the invention, therefore, the pores of the cell wall 2itself are not completely closed and can be maintained at a state nearto that prior to the covering, so that the pressure loss is considerablysmall as compared with the conventional alumina layer 103. Furthermore,the heat resistance is excellent but also the cleaning resistance isexcellent because the alumina thin film 3 covers the individual SiCparticle itself and is never peeled off from the cell wall in, forexample, the cleaning.

The pressure loss characteristic, heat resistance and cleaningresistance of the catalyst carrier according to the invention will bedescribed below.

As to Pressure Loss Characteristic

In general, the pressure loss characteristic when the exhaust gas passesthrough the cell wall as a filtering wall is considered as follows. Thatis, the pressure loss when a diesel exhaust gas passes through thecatalyst carrier (filter) constituted with the above catalyst carriercan be shown in FIG. 4. In this case, each of the resistances ΔP1, ΔP2,ΔP3 is dependent upon the cell structure of the filter and is a constantvalue Δpi=(ΔP1+ΔP2+ΔP3) not depending upon the lapse of time such asdeposition of diesel particulates and the like, which is called as aninitial pressure loss. And also, ΔP4 is a resistance when the exhaustgas passes through the deposited diesel particulates, which is a valuecorresponding to 2–3 or more times of the initial pressure loss.

The surface area of the carrier having a cell structure of 14/200 is8.931 cm²/cm³ and the density of the carrier is 0.675 g/cm³, so that thesurface of the cell wall 2 is 0.0013 m²/g. On the other hand, thesurface area of the pore in the cell wall 2 is 0.12 m²/g as measured bya mercury porosimeter, which is about 100 times of the cell wallsurface. This shows that in case of covering the surface of the cellwall with the same weight of alumina, the thickness of alumina can berendered into 1/100 in the covering of individual surface of theparticles constituting the cell wall as compared with the uniformcovering of cell wall surface.

That is, when the alumina thin film is formed by the conventionaltechnique such as wash coat, in order to coat about 3 wt % of aluminarequired for the activity of the catalyst, the thickness of aluminalayer is required to be 50 μm. In this case, resistance for passingthrough the alumina layer is plus in addition to the resistance ΔP3passing through the cell wall as the pressure loss, so that the openingis made further small and ΔP1 becomes large. As a result, the pressureloss becomes considerably large as compared with that of a filter notcontaining alumina coat. This tendency becomes more remarkable when theparticulates are deposited on the filter.

In this point, in order to coat about 3 wt % of alumina required for theactivity of the catalyst in the invention, the thickness of the aluminacoat layer formed on the surface of each SiC particle constituting thecell wall is about 0.5 μm at maximum. In this case, the resistancepassing through the cell wall ΔP3 slightly increases as the pressureloss, but the other pressure loss can substantially be ignored, so thatthe pressure loss characteristic is considerably improved as comparedwith the wash coat alumina layer.

Next, alumina has generally a high specific surface area and ispreferable to be used as a film carrying the catalyst. Particularly,since it is desired to develop a catalyst stably operating at a highertemperature and having a high heat resistance at the present, a carryingfilm of alumina is required to have a higher heat resistance accompaniedtherewith.

For this end, according to the invention, the shape of each aluminaparticle is rendered into a fine fibrous shape in order to improve theheat resistance of alumina. Thus, there is adopted a method wherein thecontact point between the alumina particles can be decreased and hencethe particle growth is controlled through the lowering of the sinteringrate to increase the specific surface area.

Namely, in the alumina thin film according to the invention, themicrosection shape indicates a transplant structure of bristling aluminaparticles in fine fibrous form, and hence the contact point between theadjoining alumina fine fibers is decreased to considerably improve theheat resistance.

And also, in the invention, Si is fed from SiC or SiO₂ existing on asurface layer of SiC during the heat treatment and plays a role ofshutting a mass transfer path to improve the heat resistance. Accordingto the inventors' studies, it has been confirmed that when SiC isintentionally treated at a higher temperature to from an oxide film, theheat resistance is further improved.

Then, the cleaning resistance will be described.

The particulate deposited on the surface of the cell wall mainlyconsists of carbon, which can be removed through oxidation by a methodsuch as burning or the like. However, there is a substance remaining asash after the burning. It is an ash formed by oxidizing a compound ofCa, Mg, Zn or the like added for giving a role as a neutralizing agentor a lubricant in an engine oil or changing into a sulfate, or an ashformed by depositing a catalyst such as CeO₂, CuO or the like previouslyincorporated in a fuel for burning carbon onto the surface of the filtertogether with particulates. It is required to clean these ashes by meansof a high pressure water or the like because they deposit during therunning of the vehicle for a long time and the pressure loss of thefilter increases. In this case, it has been confirmed that the ash canbe removed by cleaning at a pressure of not less than 30 kg/cm².

In this point, when the uniform alumina film is coated onto the surfaceof the cell wall by wash coat, the thick coat layer is existent on thesurface of the cell wall as a whole by physical adsorption, so that itis substantially peeled off by the above cleaning.

On the contrary, in the invention, alumina thinly covers the surface ofeach SiC particle and forms a chemical bond to Si fed from SiC toclosely adhered to each of the particles, so that the adhesion propertyis high and hence the resistance to cleaning is high and the durabilityas the film is strong.

FIG. 5 shows a comparison between an electron microphotograph (×10K),(×30K) when the surface of the SiC particle is covered with alumina thinfilm according to the invention and an electron microphotograph (×10K),(×30K) when the surface of the cell wall is covered with alumina filmaccording to the conventional technique. In the invention, it is clearlyseen that needle-like (fine fibrous) alumina is bristled on the surfaceof each SiC particle to indicate the transplantation structure as shownin FIG. 3( c) at first sight.

Such a structure of the alumina thin film required in the invention,i.e., the crystal structure of the alumina thin film formed by coveringthe surface of each SiC particle or the like contains at least one ofγ-Al₂O₃, δ-Al₂O₃ and θ-Al₂O₃ and fine fibrous protruding aluminaconstituting the alumina thin film has a structure that a diameter is2–50 nm, and a length is 20–300 nm, and a ratio of full length/diameteris 5–50. And also, it is favorable that the thickness of the thin filmis not more than 0.5 μm and the specific surface area of alumina is50–300 m²/g. The term “thickness of alumina thin film” used herein is anaverage of a distance from the surface of SiC particle to a most apartportion of fine fibrous protruding alumina from SiC particle surface.Moreover, the diameter of alumina is desirable to be 5–20 nm, and theaspect ratio (full length/diameter ratio) is desirable to be 10–30.

The reason why the properties of the fine fibrous protruding aluminathin film are limited to the above is based on the fact that when thelength of the fine fibrous protruding alumina is less than 20 nm, it isdifficult to ensure the surface area, while when it exceeds 300 nm, thestructure becomes brittle. And also, when the diameter is less than 2nm, it is equal to or less than the size of the catalyst such as noblemetal or the like and does not act as a carrier layer, while when itexceeds 50 nm, it is difficult to ensure the desired specific surfacearea. Furthermore, when the aspect ratio is less than 5, it is difficultto ensure the required specific surface area, while when it exceeds 50,the structure becomes brittle and there may be caused a case that thefine fibrous protrusions are bent by the cleaning operation or the like.

And also, the reason why the specific surface area of the alumina thinfilm is restricted as mentioned above is due to the fact that when it isless than 50 m²/g, the sintering of the fine fibrous protruding aluminaexcessively progresses and the durability is poor. While, when thespecific surface area exceeds 300 m²/g, the fine fibrous protrudingalumina becomes too fine and does not act as so-called carrier layer orthe structure becomes brittle. Moreover, the preferable specific surfacearea is within a range of 50–200 m²/g.

Next, the amount of the alumina thin film as a carrier film in thecatalyst carrier according to the invention is 0.5–15 wt % as an aluminaratio. When it is less than 0.5 wt %, the effect of improving the heatresistance is small, while when it exceeds 15 wt %, the pressure lossincreases and the filter function lowers. More preferably, it is 1–4 wt%.

And also, the content of silicon when the carrier in the catalystcarrier according to the invention is porous silicon carbide ispreferable to be 0.01–10 wt %. When the silicon content is less than0.01 wt %, the ability of feeding Si is deficient and the effect ofimproving the heat resistance is less, while when the silicon contentexceeds 10 wt %, the strength of the honeycomb filter lowers. Such asilicon content is favorable to be 0.01–10 wt % in the other siliconcontaining ceramics for the same reason as mentioned above, preferably0.01–5 wt %, more particularly 0.01–2 wt %.

The production method of the catalyst carrier is described below.

The characteristic of the production method according to the inventionlies in a point that the alumina thin film is formed on the catalystcarrier by sol-gel process, particularly the surface of the each ceramicparticle such as SiC or the like forming the cell wall is individuallycovered with the alumina thin film and calcined and subjected to atreatment with hot water to modify the alumina thin film into such athin film that the microstructure section indicates the transplantationstructure of bristling fine fibers of alumina.

Each step is explained in detail below.

a. Preliminary Treating Step

This step is a heating treatment at 800–1600° C. for 5–100 hours foroxidizing the surface of each silicon-containing ceramic particle suchas SiC or the like to supply Si amount in order to promote chemical bondto alumina. Of course, this step may be omitted if sufficient oxide filmis existent on the surface of the above ceramic particle. For example,the SiC sintered body itself contains about 0.8 wt % of SiO₂. They areexistent on the surface of SiC or intergranular face thereof, so thatthe supply of SiC is easily guessed. Further, there is also a purpose inthe increase of SiO₂ to improve the heat resistance. In this case, it isdesirable to heat in an oxidizing atmosphere at 800–1600° C. for 5–100hours. When the temperature is lower than 800° C., the oxidationreaction hardly occurs, while when it exceeds 1600° C., the oxidationreaction proceeds too much and the strength of the filter is lowered.The recommended conditions are 1000–1500° C. and 5–20 hours. If thiscondition is satisfied, SiO₂ sufficient to supply Si can be formed onthe surface, and the porosity and pore size of the filter areunchangeable and hence the pressure loss characteristic is not damaged.

b. Solution Impregnating Step

This step is a treatment that the surface of each ceramic particleconstituting the cell wall is covered with the thin film of alumina byimpregnating a solution of aluminum containing metal compound throughsol-gel process.

In the preparation of the solution of the above aluminum containingmetal compound, there are a metal inorganic compound and a metal organiccompound as a starting metal compound. As the metal inorganic compoundare used Al(NO₃)₃, AlCl₃, AlOCl, AlPO₄, Al₂(SO₄)₃, AlPO₄, Al₂(SO₄)₃,Al₂O₃, Al(OH)₃, Al and the like. Among them, Al(NO₃)₃ and AlCl₃ arepreferable because they are easily dissolved in a solvent such asalcohol, water or the like and is easy in the handling.

As an example of the metal organic compound, there are a metal alkoxide,a metal acetylacetonate and a metal carboxylate. Concretely, there areAl(OCH₃)₃, Al(OC₂H₃)₃ and Al(iso-OC₃H₇)₃ and the like.

As the solvent, at least one of water, alcohol, diol, polyvalentalcohol, ethylene glycol, ethylene oxide, triethanolamine, xylene andthe like is used considering the dissolution of the above metalcompound.

And also, hydrochloric acid, sulfuric acid, nitric acid, acetic acid orhydrofluoric acid may be added as a catalyst in the preparation of thesolution. Furthermore, it is effective to add Li, K, Ca, Sr, Ba, La, Pr,Nd, Si or Zr or a compound thereof to the starting material in order toimprove the heat resistance of alumina.

In the invention, Al(NO₃)₃ may be mentioned as a recommended metalcompound. Because, it is dissolved in the solvent at a relatively lowtemperature and can prepare the starting solution. And also, 1, 3 butanediol is recommended as the solvent. A first reason is due to the factthat the viscosity is appropriate and can form a gel film having aproper thickness on SiC particle at a gel state. A second reason is dueto the fact that this solvent forms a metal alkoxide in the solution andcan form a precursor for a metal oxide polymer consisting ofoxygen-metal-oxygen bond, i.e. a metal oxide gel.

The amount of Al(NO₃)₃ is desirable to be 10–50 wt %. When it is lessthan 10 wt %, the amount of alumina having such a surface area that theactivity of the catalyst is maintained for a long time can not becarried, while when it exceeds 45 wt %, the quantity of heat generationbecomes large in the dissolution and the gelation is easily caused.

Moreover, the temperature in the preparation of the impregnationsolution of the aluminum containing metal compound is desirable to be50–130° C. When it is lower than 50° C., the solubility of the solute islow, while when it exceeds 130° C.; the reaction rapidly progresses tocause the gelation and the solution can not be used as a coatingsolution. The stirring time is desirable to be 1–9 hours. The viscosityof the solution becomes stable within such a range.

In order to spread the thus adjusted metal compound solution into allpores as a space between the ceramic particles in the cell wall, theremay be adopted, for example, a method wherein the catalyst carrier(filter) is placed in the container and the metal compound solution isfilled therein and then deaerated, a method wherein the solution isflowed into the filter on one hand and deaerated on the other hand, andthe like. In this case, an aspirator, vacuum pump and the like may beused as a deaerating device. That is, air is removed off from the poresin the cell wall to fully spread the solution of the above metalcompound onto the surface of each ceramic particle.

c. Drying Step

This step is a treatment that volatile component such as NO₂ or the likeis evaporated off to gelate the solution to thereby adhere to thesurface of each ceramic particle and at the same time extra solution isremoved, which is carried out by heating at 120–170° C.×about 2 hr. Whenthe heating temperature is lower than 120° C., the volatile componentshardly evaporate, while when it exceeds 170°, the thickness of thegelated film becomes ununiform.

d. Calcining Step

This step is a calcining treatment that the residual component isremoved to form amorphous alumina, which is desirable to be carried outby heating to a temperature of 300–500° C. When the calciningtemperature is lower than 300° C., it is difficult to remove theresidual organic substance, while when it exceeds 500° C., Al₂O₃ iscrystallized and the fine fibrous protruding boehmite can not be formedby a subsequent hot water treatment.

e. Hot Water Treating Step

This step is a treatment for shaping the structure of the given aluminathin film. In this treatment, immediately after the calcined catalystcarrier is immersed in water, the particles on the surface of theamorphous alumina thin film are discharged in the solution at a solstate by deflocculating action and also boehmite particles produced byhydration are aggregated at fine fibrous protruding state and renderedinto a stable state against the deflocculation.

That is, the alumina thin film adhered to the surface of the eachceramic particle is bristled as fine fibrous form (needle-likeparticles) by the hot water treatment to form a thin film of roughsurface indicating so-called transplantation structure. Therefore, thethin film has a higher specific surface area. In general, in thesintering of alumina, the surface diffusion mainly progresses and issubjected to a phase transformation into α-alumina, at which thespecific surface area is rapidly decreased. However, when silica iscaught into the alumina particle, it is considered that the silica clogswith the pore site of alumina in the course of the heat treatment ormoves into the needle-like particle surface to control the surfacediffusion or the sintering between the particles. Therefore, viscousflowing mechanism based on the sintering from contact point between theneedle-like particles is preferential at an initial sintering of thecarrier, but it is considered that silica shuts the mass transfer pathbetween the needle-like particles to obstruct the transformation ofα-alumina at the last stage and hence the sintering does not progressand the high specific surface area is maintained.

The temperature in the above hot water treatment is desirable to be50–100° C. When the temperature is lower than 50° C., the hydration ofthe amorphous alumina thin film does not progress and the fine fibrousprotruding boehmite is not formed. While, when it exceeds 100° C., waterevaporates and it is difficult to maintain the step for a long time. Thetreating time is desirable to be not less than one hour. When it is lessthan one hour, the hydration of alumina becomes insufficient.

f. Firing Step

This step is a treatment that the boehmite produced by hydration isshaped into alumina crystal. The firing temperature is favorable to be500–1000° C. and the time is 5–20 hours. When the temperature is lowerthan 500° C., the crystallization does not progress, while when itexceeds 1000° C., the sintering excessively progresses and the surfacearea tends to be lowered.

EXAMPLES

Catalyst carriers produced under conditions shown in Table 1 (InventionExamples 1, 2, Comparative Example 1) are attached to a particulatefilter (DPF) in an exhaust gas cleaning device of a diesel vehicle toconduct a cleaning test. In this test, the pressure loss characteristic,heat resistance and resistance to cleaning are investigated to obtainresults as shown in the same table and FIGS. 4 and 5.

TABLE 1 Comparative Example 1 Example 2 Example 1 Honeycomb carrier SiCfilter SiC filter SiC filter Alumina carrying coat impregnationimpregnation wash coat Pretreatment of filter none 1100° C., 20 hr noneSiO₂ amount 0.2 wt %   3 wt % 0.2 wt % (Si amount) (0.11 wt %)  (1.0 wt%) (0.11 wt %)  Al₂O₃ amount 3.2 wt % 3.0 wt % 3.1 wt % Alumina thinfilm diameter  10 nm  6 nm — length 150 nm 120 nm — full length/ 15 20 —diameter Pressure loss FIG. 6 characteristic Heat resistance FIG. 7Resistance to cleaning no peeling no peeling at almost at 70 Kg/cm² 80Kg/cm² peeling at 10 Kg/cm²

a. As shown in FIG. 6, the invention example shows substantially thesame pressure loss characteristic as in the case of no thin film priorto the deposition of particulate (floating particular substance: PM),but the pressure loss is considerably small as compared with ComparativeExample 1 when the same gas is passed after the deposition.

b. As shown in FIG. 7, Examples 1 and 2 are small in the lowering of thespecific surface area of alumina and excellent in the heat resistance ascompared with Comparative Example 1 when the heat treatment is carriedout at the same temperature.

c. And also, Examples 1 and 2 are considerably large in the cleaningresistance as compared with the comparative example.

Moreover, FIG. 5 shows electron microphotographs (×10K, ×30K) ofparticle structure in the cell wall of the filter of Example 1 and thathaving no alumina coat. The alumina thin film according to the inventionshows the transplantation structure of bristling fine fibers.

INDUSTRIAL APPLICABILITY

As mentioned above, according to the invention, there can be provided acatalyst carrier for the purification of exhaust gas from an automobilehaving a small pressure loss, an excellent heat resistance and a goodresistance to cleaning and realized an establishment of an advantageousproduction technique therefor. Particularly, it is favorable as acatalyst carrier for cleaning the exhaust gas from the diesel engine.

1. A catalyst carrier, wherein the carrier comprises particles of asilicon-containing ceramic material and each of these particles iscovered by a film of alumina, wherein the film of alumina has a specificsurface area of 50–300 m²/g and a microscopic view thereof indicates abristled transplant structure comprising fibers having a diameter offrom 2 nm to 50 nm, a length of from 20 nm to 300 nm and a ratio oflength/diameter of from 5 to
 100. 2. The catalyst carrier of claim 1,wherein the silicon-containing ceramic material comprises a non-oxidesilicon ceramic.
 3. The catalyst carrier of claim 2, wherein thenon-oxide silicon ceramic material comprises at least one of siliconcarbide and silicon nitride.
 4. The catalyst carrier of claim 1, whereinthe silicon-containing ceramic material comprises an oxide siliconceramic.
 5. The catalyst carrier of claim 4, wherein the oxide siliconceramic comprises at least one of sialon, mullite and cordierite.
 6. Thecatalyst carrier of claim 1, wherein the carrier comprises at least oneof a porous body, a fiber-shaped body and a pellet-shaped body.
 7. Thecatalyst carrier of claim 1, wherein the carrier comprises ahoneycomb-shaped porous sintered body of silicon carbide.
 8. Thecatalyst carrier of claim 1, wherein the silicon-containing ceramicmaterial comprises a layer of SiO₂ on a surface thereof.
 9. The catalystcarrier of claim 8, wherein the amount of SiO₂ in the carrier is from0.001% to 20% by weight.
 10. The catalyst carrier of claim 1, whereinthe alumina film is present in the carrier in an amount of from 0.1% to15% by weight, based on the carrier and expressed as alumina.
 11. Acatalyst carrier, wherein the carrier comprises particles of asilicon-containing ceramic material and each of these particles iscovered by a film of alumina, and wherein the silicon-containing ceramicmaterial comprises a layer of SiO₂ on a surface thereof, an amount ofSiO₂ in the carrier being from 0.001% to 20% by weight.
 12. The catalystcarrier of claim 11, wherein the silicon-containing ceramic materialcomprises a non-oxide silicon ceramic.
 13. The catalyst carrier of claim12, wherein the non-oxide silicon ceramic material comprises at leastone of silicon carbide and silicon nitride.
 14. The catalyst carrier ofclaim 11, wherein the silicon-containing ceramic material comprises anoxide silicon ceramic.
 15. The catalyst carrier of claim 14, wherein theoxide silicon ceramic comprises at least one of sialon, mullite andcordierite.
 16. The catalyst carrier of claim 11, wherein the carriercomprises at least one of a porous body, a fiber-shaped body and apellet-shaped body.
 17. The catalyst carrier of claim 11, wherein thecarrier comprises a honeycomb-shaped porous sintered body of siliconcarbide.
 18. The catalyst carrier of claim 11, wherein the alumina filmis present in the carrier in an amount of from 0.1% to 15% by weight,based on the carrier and expressed as alumina.